In general, a wireless communication system has flexibility without the restriction of cables. On the other hand, the probability of data error occurrence upon reception is high, and the reliability is lower than in wired communication. For example, in data reception, attenuation of a received signal (hereinafter also referred to as received data) makes the wireless signal extremely feeble and the signal to noise ratio (SN ratio) low. For this reason, demodulation errors occur at high probability. In addition, since the radio wave reflected by various objects reaches the destination node while being delayed from the direct wave, the destination node receives the plurality of radio waves having different delay times and strengths. This sometimes causes a signal distortion in the received data, resulting in data errors. That is, when a signal having a low SN ratio and containing a distortion is demodulated on the destination node side, data errors occur at high probability. To prevent this, data protection is done in some cases using, for example, an error correction code.
If the distance between the source node and the destination node is long to some extent, it is impossible to ensure the necessary SN ratio on the destination node side, and a relay apparatus needs to be provided between the source node and the destination node (Japanese Patent Laid-Open No. 2006-54675). The relay apparatus receives data (wireless signal) transmitted from the source node and demodulates and decodes the data using an error correction code. The relay apparatus then re-encodes and re-modulates the data and transmits the re-modulated data to the destination node. Since the relay apparatus eliminates the influence of noise and distortion, it is possible to relay-transmit, to the destination node side, data having almost the same quality as that of the original data transmitted from the source node.
Redundancy transmission by diversity is known as a technique of improving the reliability of wireless communication. In the redundancy transmission technique, a plurality of communication paths from a source node to a destination node are provided. Data identical to that transmitted from the source node are transmitted to the destination node using the plurality of different communication paths.
When a plurality of communication paths (to also be referred to as transmission paths hereinafter) are used, even in case of communication breakdown in one of the communication paths, data transmission/reception can be done via the remaining communication paths, and the communication quality from the source node to the destination node can be maintained. Hence, use of redundancy transmission prevents loss of transmission data caused by breakdown of a communication path and also obviates resend control processing for recovering data loss.
For this reason, redundancy transmission is often used in, for example, a system which requires very high reliability or a synchronous data transfer system which prohibits use of resend processing. In redundancy transmission, the destination node side receives a plurality of data via the plurality of communication paths and then combines and decodes them. Hence, a diversity effect can be attained.
There is also known a technique of transmitting data without performing the above-described decoding/re-encoding (A Practical Scheme for Wireless Network Operation, IEEE Trans. on Comm., VOL. 55, NO. 3, March 2007). In this technique, when transmitting data from a source node to a destination node, a relay apparatus transmits received data to the next communication node without decoding/re-encoding the data. This increases both the communication capacity and the channel capacity.
Even when data protection using an error correction code is performed to reduce data errors, the relay apparatus cannot correct all errors if the received data contains errors in number beyond the correction capability of the error correction code. In this case, the data containing the errors is relay-transmitted to the destination node side.
The principle of data error occurrence in wireless communication will be explained here with reference to FIG. 32A. FIG. 32A shows an outline of a modulated signal in BPSK (Binary Phase Shift Keying).
In the BPSK, phase modulation is executed such that, for example, when a data symbol to be modulated is “0”, the phase of the carrier signal becomes 0° (point A), and when the data symbol is “1”, the phase of the carrier signal becomes 180° (point B). A destination node maps the modulation point of a received BPSK signal on the in-phase axis, and acquires the position on the in-phase axis as a metric value. If the metric value is positive, the received data symbol is acquired as “0”. If the metric value is negative, the received data symbol is acquired as “1”.
A BPSK modulated signal which has just been modulated in a source node rarely contains noise components. Hence, in this modulated signal, the metric values of data symbols “0” and “1” are almost +1.0 and −1.0, respectively. However, a wireless signal that has passed through a communication path contains superimposed noise or distortion. In this case, metric values generated in the destination node (or relay apparatus) have a Gaussian distribution as shown in FIG. 32A because of the influence of noise. If the noise components contained in the received data (received signal) are large, the distribution extends widely. For example, if the distribution in the region where the sign of the metric value is positive extends into the negative region beyond the metric value of 0, the destination node erroneously determines a data symbol “0” as “1”. A reverse phenomenon also occurs. This failure generates data errors in the destination node.
An outline of redundancy transmission using three communication paths will be described next with reference to FIG. 32B. Note that the numerical values in FIG. 32B indicate metric values at given points. Since the influence of noise and distortion in a communication path varies over time, each metric value in FIG. 32B represents an instantaneous value of a transmission bit of interest.
The transmission data bit is “0”, a source node 50 modulates and transmits the bit “0” as a metric value of +1.0. The signal transmitted from the source node 50 suffers the influence of attenuation and distortion on a communication path. Hence, each relay apparatus receives a signal whose metric value has varied from +1.0 upon transmission. For example, a relay apparatus 51 receives a signal having a metric value of +0.4. A relay apparatus 52 receives a signal having a metric value of +0.5. A relay apparatus 53 receives a signal having a metric value of −0.1. The relay apparatuses 51 and 52 decode the signals into the normal value (bit “0”) and then modulate and relay-transmit them because the metric values are positive. However, the relay apparatus 53 which has received the metric value of −0.1 relay-transmits the data bit as the wrong metric value of −0.1.
As in the communication path from the source node to each relay apparatus, the wireless signal is affected by noise and distortion even in the communication path from each relay apparatus to the destination node. A destination node 54 receives metric values of +0.3, +0.4, and −1.1 as the received data from the relay apparatuses, and combines and decodes them. Assume that the destination node 54 executes combination decoding by adding the metric values (the three metric values are added to generate a combined metric value). In this case, the combined metric value is −0.4, that is, negative. For this reason, the destination node 54 determines the combined received data symbol erroneously as “1”.
As described above, if the relay apparatus re-encodes data containing errors, the data containing superimposed errors is relay-transmitted. As a consequence, data errors occur in the destination node even after combination decoding, and no sufficient diversity effect by combination decoding can be obtained.